The galactic corona of spiral galaxies like M31 and M33 is a source of synchrotron radiation.
One cannot know that from popular cosmology which ignores plasma.
The terms galactic corona and gaseous corona have been used in the first decade of the 21st century to describe a hot, ionised, gaseous component in the galactic halo of the Milky Way. A similar body of very hot and tenuous gas in the halo of any spiral galaxy may also be described by these terms.
This coronal gas may be sustained by the galactic fountain, in which superbubbles of ionised gas from supernova remnants expand vertically through galactic chimneys into the halo. As the gas cools, it is pulled back into the galactic disc of the galaxy by gravitational forces.
A possible first impression: this description is nonsense!
Hot gas is plasma. If it cools it might remain plasma.
Hot gas does not rise up in a galaxy like hot gas rising up a chimney from the fire below. A galaxy does not have hot and cold gases blowing about in the near vacuum of space, with hot rising and cold falling. On Earth cold gas falls due to gravity. In a galaxy, ionized gases move by electromagnetic forces not by gravity.
The reality of the galactic corona was revealed by an astronomer at an observatory in France in 2013 who used a 600mm telescope with its spectrograph to capture a slice of the galactic corona spectrum for M31 and M33.
For both galaxies:
The entire spectrum is possibly shifting. A consistent shift throughout indicates the light source is moving as revealed by the Doppler effect.
If only the absorption lines are shifting then only those atoms are showing the Doppler effect.
The flat spectrum of synchrotron radiation is difficult to determine whether the two "ends" are shifting the same.
Distant galaxies have a hydrogen absorption line with red shift. Only that line shifts, not the entire spectrum. Therefore the absorption line shift comes from hydrogen atoms in the IGM.
The author of these observations in France noted Hubble provided no reference data for Hubble's 1936 velocity for M33. That is the big problem for independent analysis: lack of public data.
The slit spectra clearly show the corona generates synchrotron radiation. The 3 stars spectra (in blue, green, red) have the distinctive hump of thermal radiation for the hottest star. Star spectra slope down to around 700nm or 7000 Angstroms, or infrared.
A spiral galaxy has a bulge of stars around the central Z-pinch where the galactic birkelund current pair bends and splits for the filaments out the spiral arms. These filaments generate the magnetic field which maintains some structure in the arms.
unfortunately, it impossible to conclusively determine whether those calcium atoms are moving with the corona or just in the line of sight.
However the similar, but not identical, blue shifts among all these objects, M31, M33, NGC 604, indicates either the objects are moving with similar, different approach velocities or the calcium atoms are.
Probably all are moving with their blue shift velocities.
The galactic corona as a source of synchrotron radiation is an enigma.
The solar corona is the electric discharge between the photosphere and the solar environment extending to the heliopause.
There is no apparent 'surface' in a galaxy like a photosphere.
The galactic corona must be the electric discharge between a plasma envelope around the central bulge, and the 'galactic-pause' for the galaxy within its cluster. This mechanism is not clear.
The spectrum of the corona slice is clearly not of a star. it is clearly electrical (not from a gas).
A separate topic, but related...
An elliptical galaxy has no suggested corona.
However it is my personal opinion they might.
Distant galaxy clusters with giant elliptical galaxies will sometimes have luminous arcs as portions of a sphere.
These are always dismissed as from a gravitational lens. These arcs must be real so I suggest a giant elliptical galaxy might exhibit a galactic corona discharge, similar to a spiral galaxy. Both have a plasmoid residing at their center surrounded by much stuff.
These arcs were the subject of my October 2, 2019 post: Collection of Rings and distortions, with images.
I will post this while I consider possible conclusions.
For now, we know more about a galactic corona than provided by Wikipedia.